Flame resistant fabrics (also variously referred to as “FR”, “fire-resistant,” “flame-retardant,” and “fire-retardant” fabrics) are fabrics that, once ignited, tend not to sustain a flame, when the ignition source is removed. Considerable research has been directed toward the development and improvement of flame-resistant fabrics for use in various products, including clothing and bedding. Flame-resistant clothing is often worn by workers involved in activities, such as industrial manufacturing and processing (such as oil, gas, and steel industries), fire-fighting, electrical utility work, military work, and other endeavors that entail a significant risk of being exposed to open flame, flash fire, momentary electrical arcs, and/or molten metal splash. Non flame resistant work clothes can ignite and will continue to burn even after the ignition source has been removed. Untreated natural fabrics will continue to burn until the fabric is totally consumed and non-flame resistant synthetic fabrics will burn with melting and dripping, causing severe contact burns to the skin. A significant portion of severe and fatal burn injuries are due to the individual's clothing igniting and continuing to burn, not due to the initial exposure itself. Abrasion resistance of protective fabrics is also important, as garments that have developed failures, such as holes and rips, can compromise the protective properties of the fabric.
Flame-resistant fabrics include both fabrics that are treated to be flame-resistant as well as fabrics made from inherently flame-resistant fibers. The former types of fabrics are not themselves flame-resistant, but are made flame resistant by applying to the fabric a chemical composition that renders the fabric resistant to flame. These types of fabrics are susceptible to losing their flame resistance with repeated launderings with hypochlorite bleach. Hypochlorite bleach attacks the finish and reduces the flame-resistant properties of the fabric. In contrast, inherently flame-resistant fabrics do not suffer from this drawback because they are made from fibers that are themselves flame-resistant. The use of flame resistant clothing provides thermal protection to areas of the body covered by the garment. The level of protection typically rests in the fabric weight, construction, and composition. After the ignition source is removed, a flame resistant garment will self-extinguish, limiting the body burn percentage.
Flame-resistant fabrics may contain a low percentage of natural fibers and have limited comfort properties, such as water absorption and breathability. Flame-resistant fabrics are most often worn in work environments, where comfort, including absorption of sweat from the skin, is an important performance factor, especially in extreme conditions such as firefighting. Combining some percentage of natural hydrophilic fibers with FR fibers may provide some improvement in comfort and moisture wicking, however this typically comes at a loss of FR performance properties. Most FR fibers, including aramid fibers, are hydrophobic and do not provide high comfort performance. Adding a high concentration of hydrophilic fibers, however, may negatively impact moisture management properties and/or fire resistance properties. In addition, garments made from fabrics having high percentage content of hydrophilic fibers may become oversaturated with moisture, such as from sweat, and cause additional burns, when expose to a high temperature.
In addition, fabrics made with a high percentage of aramid fibers, including meta-aramid and/or para-aramid, fibers are typically stiff, have poor softness or drape properties, and are generally uncomfortable to wear. The softness of fabrics made with a high percentage of aramid fibers may be improved by repeated washings but tend to become more hydrophobic. Therefore, many industrial workers, pilots, and emergency responders repeatedly wash garments made with high percentages of aramid fibers to increase comfort, even washing new garments many times prior to the initial use. Unfortunately, many of these garments are made with hydrophobic and/or hydrophilic coatings that can lose effectiveness with repeated washings. Therefore, washed treated garments may have improved softness but decreased moisture management properties.
Various types of inherently FR fibers have been developed, including modacrylic fibers (e.g., modacrylic fibers sold under the PROTEX name from Kaneka Corporation of Osaka, Japan, and Tairylan sold by Formosa Plastics of Taiwan). Acrylic based FR fibers sold under the name PyroTex, (Hamburg, Germany), aramid fibers (e.g., meta-aramid fibers sold under the NOMEX name and para-aramid fibers sold under the KEVLAR name, both from E.I. DuPont de Nemours and Company of Wilmington, Del.), FR rayon fibers (sold under the Lenzing FR name, from Lenzing Group, Austria), oxidized polyacrylonitrile fibers, and others. It is common to blend one or more types of FR staple fibers with one or more other types of non-FR staple fibers to produce a fiber blend from which yarn is spun; the yarn then being knitted or woven into fabrics for various applications. In such a fiber blend, the FR fibers render the blend flame-resistant even though some fibers in the blend may themselves be non-FR fibers, because, in the case of antimony- and halogen-filled fibers, when the FR fibers are exposed to heat and flame they release non-combustible gases that tend to displace oxygen and thereby extinguish any flame. In addition to char formation, and having high Oxygen Limiting Index (LOI), many FR fibers are poor conductors of heat. In the case of non-filled FR fibers the high percentage of FR fibers form char, or exhibit other characteristics which provide wearer protection.
In addition to the above-noted performance specifications of fabrics, other properties are also important if a fabric is to be practical and commercially viable, particularly for clothing. For instance, the fabric should be durable under repeated industrial and home launderings and should have good abrasion-resistance. Furthermore, the fabric should be comfortable to wear. Unfortunately, many of the FR blends are not comfortable under typical environmental conditions. In such cases, wearers tend to be less likely to be compliant and thereby decreasing the probability that the wearer will continue to use the garment as intended. Thus, it is beneficial if a FR fabric exhibits good moisture management properties, i.e., ability to wick away sweat and dry quickly, so that the wearer does not become overheated or chilled, and/or the fabric does not irritate the wearer's skin.
Furthermore, many inherently FR fibers and especially most aramid type FR fibers are not dye accepting. It is desirable in most applications to have FR fabric that is dye accepting or “printable”. In some cases, fibers may be purchased that are producer colored, however this limits the color options available to the fabric manufacturer.
Selection of a fiber blend to meet a plurality of the requirements as described, while being affordable is a constant challenge. Some (FR) fibers and especially inherently FR fibers that are thermally shrink resistant, as defined herein, are relatively expensive and incorporating a high percentage of these fibers into a yarn and fabric may be cost prohibitive for many applications.
Woven FR fabrics are well suited for meeting the requirements of the FR test protocols, including NFPA 2112 and especially the thermal shrinkage tests. Woven fabrics are relatively tight, having little void volume between yarns, therein reducing the propensity to thermally shrink. Other types of fabric structures, such as knits, may be more comfortable to wear as they typically have higher porosities. However, knit fabric may not meet the thermal shrinkage requirements. The yarns in a knit fabric are looped and therefore not as restrained as yarns in a conventional woven fabric and therefore can shrink more.
One of the hazards to which workers are exposed is arc flash, which is an explosive release of energy caused by an electrical arc. An arc flash results from either a phase to ground or a phase to phase fault caused by, for example, accidental contact with electrical systems, accumulation of conductive dust, corrosion, dropped tools, and improper work procedures. During an arc flash, the temperature can reach 35,000° F., and exposure to an arc flash can result in serious burn injury and death.
Arc rating is the value of energy necessary to pass through any given fabric to cause with 50% probability a second or third degree burn. This value is measured in calories/cm2. The necessary arc rating for an article of clothing is determined by a Hazard/Risk Assessment and the resulting Hazard Risk Category, and is typically measured in terms of arc thermal performance value (ATPV) or energy break open threshold (EBT). For a fabric to be considered useful in most job situations, an arc rating of at least 8 calories/cm2 is required. In the trade, fabrics that meet both an arc rating of at least 8 calories/cm2 and NFPA 2112 are considered dual hazard. Arc rating determines the protective characteristics of the fabric and the higher the arc rating value the greater the protection.
The primary purpose of FR fabric is to resist ignition (as tested by ASTM D-6413, also known as Vertical Flame Test). If fabric is ignited by an arc flash, flash fire, molten metal, and like, the hazard to the wearer instantaneously escalates, because the fire will last much longer than the initial hazard, will typically burn the victim over a much larger body surface area and more deeply, and is more likely to result in airway and lung damage. By not continuing to burn after the initial hazard is over, FR fabric limits burn injury to, at most, only the body surface area directly impacted by the hazard. Limiting the total body surface area greatly improves survival for the victim. The second goal of FR fabric is to insulate the wearer from the thermal hazard, thus reducing or eliminating any second or third degree burn through the fabric, even in areas directly impacted by the hazard. Arc rating measures the protective value of the fabric to this hazard.
Since all arc rated fabrics are also FR, they will provide some measure of protection in a flash fire. However, arc rating is not predictive of flash fire performance, which must be separately tested. The consensus standard for flash fire is NFPA 2112 Standard on Flame-resistant garments for protection of industrial personnel against flash fire. NFPA 2112 lists multiple requirements for certification to the standard including ASTM F 1930: Standard Test Method for Evaluation of Flame Resistant Clothing for Protection Against Fire Simulations Using an Instrumented Manikin. A fabric passes this test if it records less than 50% in second and third degree body burns in a three-second flash fire, expressed as a percent body burn. The lower value the better the fabric performs.
To improve arc rating, garment manufacturers adjust the fabric weight, composition, and construction. For example, increasing the weight of the fabric typically improves the arc rating. Unfortunately, increased fabric weight can make garments uncomfortable, bulky, and stiff and may lead to non-compliance by the wearer. Also, for lighter weight fabrics, such as those used in undergarments, it may not be possible to achieve the arc rating and flash fire resistance, required by workers in high hazard fields, such as utility workers, industrial works, fire fighters, and military personnel. Thus, there exists a need for lightweight, dual hazard (arc rated and flash fire resistant) fabrics, which also provide superior moisture management properties and strength properties to improve wearer compliance. The fabrics, garments, and articles of the present invention are directed toward these, as well as other, important ends.